Cornbread Casserole

2 large onions, chopped
6 tablespoons butter or margarine
2 eggs
2 tablespoons milk
2 (17 ounce) cans cream-style corn
1 (16 ounce) package corn muffin mix or 2 package Jiffy mix
1 cup sour cream
2 cups shredded sharp Cheddar cheese

Preheat oven to 425 degrees F. Grease a 9 x 13-inch baking dish.

In a medium skillet saut